chapt04gh - Chapter 4 Continental Margins and Ocean Basins...

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Unformatted text preview: Chapter 4 Continental Margins and Ocean Basins Bathymetry of Sea Floor Continental Margins and Ocean Basins Submarine Canyons Hydrothermal Vents Trenches Bathymetry: a map of the ocean floor Early bathymetric studies were often performed using a weighted line to measure the depth of the ocean floor. Echo sounding LIDAR Multibeam Systems Satellite Altimetry Vw ~= 1500 m/s (Pres, Temp, Salinity) Echo Sounders Bounce Sound off the Seabed 1 2 LIDAR: Light Imaging Detection And Ranging Vw ~= 1500 m/s (Pres, Temp, Salinity) Va ~= 344 m/s (Pres, Temp, other) h, ft Shinnecock Inlet Moriches Inlet 3 4 Multi-beam echo sounder Using satellite measurements to map the ocean floor 5 6 1 Gravitational Anomalies: Bathymetry of ocean floor-ridges, shelves floor- Topography of Ocean Floors Cross section of the Atlantic ocean basin and the continental United States, showing the range of elevations. Ocean depth is clearly greater than the average height of the continent, but general range of contours is similar. 7 8 An active margin PeruChile Plate Trench boundary Narrow continental shelf Continental margin Sediment Deep-ocean basin Continental margin Submarine canyon profile (cut through continental shelf) Pacific Ocean Nazca Plate Plate boundary Andes Mountains Broad South America continental shelf Atlantic Ocean South American Deep basin Plate Afr Plate movement ican Plate A passive margin Plate movement Plate movement Mid-Atlantic Ridge (spreading centers, shallow earthquakes) Continental shelf Continental slope Oceanic ridge Continental rise Continental crust (granitic) Oceanic crust (basaltic) Sediment Continental crust (granitic) Oceanic crust (basaltic) Subduction zone (deep and shallow earthquakes) Atlantic = Passive Margin little/no geologic activity Pacific = Active Margin geologic activity Asthenosphere 9 10 The ocean floor can be classified as (a) Continental Margins the submerged outer edge of a continent (b) Ocean Basin the deep seafloor beyond the continental margin Continental Margins There are two types of continental Margins (a) passive or trailing margins: margin of continent that moves away from spreading center Atlantic-style margins (also Artic Ocean, Antarctica and Indian Ocean). Very little volcanic or earthquake activity is associated with passive margins. (b) active or leading margins: plate boundary located along a continental margin ocean trenches where there is subduction of oceanic lithosphere narrow, steep, with volcanic mountains (West Coast of the Americas). Active margins are the site of volcanic and earthquake activity. 11 Region where continental crust meets oceanic crust Continental Shelf Shelf Break Continental Slope Continental Rise 12 2 Continental Shelves Gently sloping (~0.5 degrees) Depositional environments Average width 65 km (40 miles) Average depth 130 m (430 feet) Narrow along Active margins Wide along Passive margins 13 Continental Shelves: broad shallow extension of the continents (~75km wide) Regions of deposition (rivers, glaciers, scrapped marine deposits, calcium carbonate) Large bedform features, reworked by tides, storms, waves 14 The Flandrian Transgression Current sea level rise which began approximately 18-19,000 years ago (during latest Pleistocene time and continuing progressively during Holocene time to the present). This rise in sea level is directly related to the melting of continental polar and mountain piedmont glaciers. During the "climax" of the Wisconsin glacial advance (lowstand) sea level was anywhere between 70 to 150 meters below its current level Shelf Break = the outer edge of the continental shelf Shoreline Retreat During The Flandrian Transgression -50 m -40 m -30 m -20 m 15 -10 m 0m 16 30 kilometer wide band of sand ridges on the middle continental shelf represent a broad band of degraded and submerged barrier islands formed between 14,000 and 8,000 years before present (Stubblefield, et al. 1983) Shelf currents are actively reworking the barrier sands into ridges It has been in the last 4000-6000 years that the majority of modern coastal barrier islands and tidal wetlands have developed. 109,868 to 517,948 yd3/yr of sediment may be coming from offshore, however the exact mechanism for the material transport into the littoral zone has not been determined (Schwab et al., 1999) 17 18 3 19 20 Anatomy of a passive margin Continental Margins Shelf Break Edge of the continental shelf Change in slope (~ 140 m) Continental Slope deep sediments accumulate here, thickness varies Extends from break to ocean basin Steep (3 6 degrees) As high as 25 degrees Little/no deposition 22 21 Continental Slopes: continental crust thins into oceanic crust steep (~20km, 1-25 degrees), 5deg Pacific, 3deg Atlantic extend to depths between 1500-4000 m Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope These are features of some continental margins. They cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment. 23 24 4 Submarine Canyon Steep V shape channel, incised in the continental slope (and shelf) Created by Rivers during the last low stand (some) Turbidity currents Turbidity Currents Fast moving avalanches of mud and sand scour slopes Form turbidite deposits 90 km/hr (56 mi/hr) Turbidite bed ancient deposit, exposed to erosion, graded deposits: largest particles at bottom 25 26 Continental Margins Continental Rise Base of the continental slope slope 0.5 1 degree Depositional environment Formed by: Turbidity currents Underwater landslides 27 28 Continental Rise: Fan like deposit where the continental slope intersects the abyssal plains Formed by turbidity currents Features of the Sea floor Oceanic Ridges Hydrothermal Vents Abyssal Plains and Abyssal Hills Seamounts and Guyots Trenches and Island Arcs Seafloor: 4000 6000 m water depth, 30% of the Earth's Seafloor: Earth' surface Abyssal Plain: vast, flat plain extending from the base of the continental slope. Ocean Basins: sections of the abyssal plain separated by continental margins, ridges, and rises. 29 5 Basins Sea floor features 31 32 Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are Flatcalled guyots Abyssal hills are flat areas of sediment-covered ocean floor found sedimentbetween the continental margins and oceanic ridges. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges. 33 ridges. Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows) is heated as it descends toward the hot magma magma chamber, leaching sulfur, iron, copper, zinc, and other materials from materials the surrounding rocks. The heated water (red arrows) returning to to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor. 34 Mid-Ocean Ridges and Rises Mid- Major ocean trenches An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean. 35 Trenches are arc-shaped depressions in the ocean floor caused by the arcsubduction of a converging ocean plate. Most trenches are around the edges of the active Pacific. Trenches are the Trenches deepest places in Earth's crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than Earth' the adjacent basin floor. The ocean's greatest depth is the Mariana Trench ocean' where the depth reaches 11,022 meters (36,163 miles) below sea level. 36 level. 6 Bathymetric devices used to study seabed features Chapter 4 - Summary Good Stuff Construct a transect (cross-section) across the Atlantic Ocean starting from the east coast of the United States. Be sure to label all of the bathymetric features and type of margin. Construct a transect (cross-section) across the Pacific Ocean starting from the west coast of the United States. Be sure to label all of the bathymetric features and type of margin. What are the noticeable differences between the two types of margins? include multibeam echo sounder systems and satellites that use sensitive radar for altimetry. Seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. Near shore, the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. The transition to basalt marks the edge of the continent and divides ocean floors into two major provinces, The submerged outer edge of a continent is called the continental margin. The deep-sea floor beyond the continental margin is deepcalled the ocean basin Features of the continental margins include continental shelves, continental slopes, submarine canyons, and continental rises. Features of the deep-ocean basins include oceanic deepridges, hydrothermal vents, abyssal plains and hills, seamounts, guyots, trenches, and island arcs. guyots, 37 38 7 ...
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This note was uploaded on 07/18/2011 for the course EARTH SCIE 180 taught by Professor Frankbuonaiuto during the Spring '11 term at CUNY Hunter.

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